For example, a vehicle having a battery mounted thereon uses an air intake duct to ventilate or cool the space containing the battery. A sound emitted by a suction apparatus adjacent to one opening of the air intake duct may leak as noise from an opposite opening through the duct. For this reason, air intake ducts for batteries are often required to have sound absorption properties. For example, foam ducts, which are formed of foam resins, are being used as such air intake ducts.
However, the fact is that foam ducts do not necessarily have sufficient sound absorption properties and have yet to produce effects that prevent noise. For this reason, the development of a sound absorption duct that can produce higher sound absorption properties is being waited for.
In the field of air-conditioning ducts, there have been known air-conditioning ducts including a foam resin layer for improving heat insulation properties and preventing condensation or the like. For example, Patent Literature 1 discloses a duct in which the outer peripheral surface of a duct main body serving as a gas passage is coated with a coating layer wall. In this duct, the coating layer wall is formed of a heat-shrinkable foam resin sheet, and the outer peripheral surface of the duct main body is coated with a heat-shrunk heat-shrinkable foam resin sheet.
Patent Literature 2 discloses an air-conditioning flexible duct that is connected to air-conditioning apparatuses, such as an air-conditioning chamber, an air-conditioning elbow, an air outlet, and a ventilation opening, and has a ventilation passage through which a wind circulates. In this air-conditioning flexible duct, a heat insulation material is fixed to the outer surface of a duct main body in the attachment use state thereof, and a heat-shrinkable film is closely pasted to the outer surface of the heat insulation material.
For the ducts disclosed in Patent Literatures 1 and 2, the foam resin sheet or heat insulation material pasted to the outer surface of the duct is believed to have sound absorption properties. Accordingly, these ducts are expected to have a certain level of sound absorption properties.
Patent Literatures 3 and 4 disclose ducts in which a foam sheet or foam is pasted to the inner surface. Specifically, Patent Literature 3 discloses an air guide duct which is disposed in a vehicle interior member, communicatively connects the air outlet of the vehicle interior member and the air feeding opening of an air-conditioning unit mounted on the vehicle, and guides a conditioned air from the air-conditioning unit to the air outlet. This air guide duct is provided with a shape required for a duct and is composed of an outer layer sheet having stiffness which allows the duct to hold the shape and an inner layer sheet which is pasted to the entire inner surface of the outer layer sheet and which performs sound absorption and heat insulation functions.
Patent Literature 4 discloses an automobile duct which is a multilayer body having an elastic foam on one side thereof. This automobile duct is formed by folding a multilayer body that has a hinge approximately in the center and flanges on both sides and has a desired recessed shape, at the hinge with the elastic foam inside and then fixing the flanges in an oppositely contacting manner.
In the ducts disclosed in Patent Literatures 3 and 4, the foam or the like is pasted to the inner surface. Further, Patent Literature 3 mentions sound absorption.